1. Field of the Invention
This invention relates generally to a glow plug to be used for preheating an auxiliary combustion chamber or combustion chamber in a diesel engine, and more particularly to a glow plug for diesel engines comprising a ceramic heater that permits after glowing for long hours.
2. Description of Prior Art
In general, a diesel engine has poor starting properties at low temperatures. To assist the diesel engine in starting, therefore, a glow plug is usually provided in an auxiliary combustion chamber or combustion chamber to raise the intake air temperature or for use as an ignition source, the heat being generated by applying electricity to the plug. The glow plug is usually of a sheathed heater type constructed by filling a metallic sheath with heat-resistant insulating powder and embedding a heater coil, made of ferrochrome, nickel, etc., in the powder. In addition to this, a ceramic heater type is also known, as disclosed in Japanese Pat. Laid-open No. 41523/1982, which comprises a heating wire, made of tungsten, etc., which is embedded in an insulating ceramic material, such as silicon nitride. The ceramic heater type has been widely used in recent years because it has a better heat transfer efficiency and an excellent heat generating performance since it becomes red hot in a short period of time during heating, compared with the sheathed heater type which involves indirect heating by means of the heat-resistant insulating powder and the sheath.
The glow plug of the ceramic heater type, however, has a metallic heating wire, made of tungsten, etc., embedded in the inside of an insulating ceramic material, such as silicon nitride. The different coefficients of thermal expansion of both the members, the sharp temperature rise during heating and the repeated use of the heater tends, to deteriorate the durability of the ceramic heater and cause problems in the reliability, including heat resistance, of the ceramic heater. This also results in increased manufacturing costs.
To solve this problem, a ceramic heater construction which employs as a heating wire an electrically conductive ceramic material having a coefficient of thermal expansion substantially equal to that of an insulating ceramic material has been proposed in Japanese Pat. Laid-open Nos. 9085/1985 and 14784/1985. However, both the proposed glow plugs still have problems in terms of both construction and function, and therefore have not been put into commercial application.
For example, the former, having a construction that an electrically conductive ceramic material as a heating element is embedded into an insulating ceramic material, has better thermal conductivity than the sheathed type, but it involves a number of problems such as a poor quick-heating function because of the indirect heating type and the difficulty in molding.
The latter, on the other hand, has a quick-heating function due to its heating element exposed to the surface of the heater, but the fact that its heating element is a laminated structure of U-shaped members, with both ends being led to the rear end of the heater, makes the electrode take-off construction complex, leading to increased manufacturing costs.
To overcome these problems, the present applicant of this invention invented, filed and disclosed as the Japanese patent application No. 9933 of 1986 a glow plug for diesel engines in which a rod-shaped ceramic heater supported by the tip of a holder is composed of a U-shaped heating portion and a pair of lead wires extending backwards from the both ends of the U-shaped heating portion; both being formed integrally by an electrically conductive ceramic material; the outside surface of one lead wire being held via an electrically conductive layer, and the other lead wire held by joining to the holder via an insulating layer.
This previous invention, in which a heating portion is formed solely by an electrically conductive ceramic material containing no foreign matter, has high reliability in terms of heat resistance, and is excellent in durability such as heating characteristics, despite the repeated thermal stresses applied during use. The previous invention is also beneficial in moldability, contributing to a reduction in manufacturing costs. In addition, the previous invention has a quick-heating function as the heater tip can be quickly red-heated by the heating portion comprising an electrically conductive ceramic material exposed to the heater surface.
In a glow plug having the aforementioned construction, the space in the holder is led to the engine combustion chamber, etc. facing the ceramic heater by a slit formed in the longitudinal direction of the ceramic heater. Consequently, it is necessary to prevent the combustion pressure developed at the time of explosion in the combustion chamber from leaking to the outside. To this end, the ceramic heater of the previous invention employs a closing member made of a ceramic material, such as alumina or mullite, as a means for closing the gap or slit formed between the components of the ceramic heater. That is, a closing member made of a ceramic material, such as alumina or mullite, is inserted into the silt and integrally bonded together using glass paste as an adhesive. With this bonding means, however, pores may be caused due to the use of glass paste. This could result in imperfect air-tightness around the bonded portion, causing carbon, oil, fuel, etc. to infiltrate into the space inside the holder, posing problems, such as corrosion of internal metallic lead wires and shortcircuiting in extreme cases. The use of glass paste of a high viscosity makes automation difficult, presenting an obstacle to the improvement of productivity. In recent years, glow plugs of this type are required to have higher durability to cope with the increased operating temperature associated with the improved starting properties of diesel engines and the increased use of turbochargers. As a result, the aforementioned means for closing the slit can no longer satisfy such stringent requirements.
In the above-mentioned ceramic heater, the most commonly used electrode bonding method for electrically connecting the ceramic heater to the power source is coating the bonding surface of a glow plug with Ni powder paste, heat-treating the paste in a vacuum at 1,150.degree. C. for 30 minutes to form a metallized layer, and brazing on the metallized layer an electrode for connection to a metallic lead wire. With this method, however, the electrode formed tends to be separated due to the low bonding strength of the metallized layer formed on the bonding surface of the glow plug. This process is low in reliability and involves long-hour heat treatment, leading to high manufacturing costs.